Chip-scale package

ABSTRACT

A method for manufacturing a semiconductor package that includes forming a frame inside a conductive can, the frame being unwettable by liquid solder.

RELATED APPLICATION

This application is based on and claims the benefit of U.S. ProvisionalApplication Ser. No. 60/673,160, filed on Apr. 20, 2005, entitled SOLDERMASK INSIDE DIRECT FET CAN, to which a claim of priority is hereby madeand the disclosure of which is incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to power semiconductor packages andmethods of manufacturing power semiconductor packages.

Referring to FIGS. 1-4, a package 10 according to the prior art includesa conductive can 12, and a power semiconductor die 14. Can 12 istypically formed with an electrically conductive material such as copperor a copper-based alloy, and may be coated with silver, gold or thelike. Die 14 may be a vertical conduction type power semiconductorMOSFET having its drain electrode 16 electrically and mechanicallyattached to an interior surface of can 12 by a conductive adhesive 18such as solder or a conductive epoxy (e.g. silver epoxy). Sourceelectrode 20, and gate electrode 22 of die 14 (which are disposed on asurface opposite to the drain electrode) each includes a solderable bodywhich facilitates its direct connection to a respective conductive pad24, 26 of a circuit board 28 by a conductive adhesive (e.g. solder orconductive epoxy) as illustrated by FIG. 4. Note that die 14 furtherincludes passivation body 30 which partially covers source electrode 20and gate electrode 22, but includes openings to allow access at least tothe solderable portions thereof for electrical connection. Further notethat in package 10 conductive can 12 includes web portion 13 (to whichdie 14 is electrically and mechanically connected), wall 15 surroundingweb portion 13, and two oppositely disposed rails 32 extending from wall15 each configured for connection to a respective conductive pad 34 oncircuit board 28. Also, note that die 14 is spaced from wall 15 of can12; i.e. wall 15 surrounds die 14. Thus, a moat 36 is present betweendie 14 and wall 15.

In a package according to the prior art, source electrode 20, and gateelectrode 22 are soldered down by the user. Specifically, the userapplies solder to, for example, the pads of a circuit board, and theelectrodes of the die are attached to the pads by the solder so placed.

A package as described above is disclosed in U.S. Pat. No. 6,624,522.

To fabricate a package as described above, a solder is applied to drainelectrode 16 of die 14, die is placed inside can 12, and the solder isreflown. Alternatively, solder is applied to the interior surface of webportion 13 of can 12, drain electrode 16 of die 14 is placed on thesolder, and the solder is reflown. In each case, once the solder isreflown, there is a possibility that die 14 may move from its positionas placed or become misaligned relative to its orientation as placed. Asa result, the quality of the final product may be adversely affected.

SUMMARY OF THE INVENTION

In a process according to the present invention, a frame is formedinside a conductive can which is not capable of being wet by solder inliquid phase. The frame so formed defines a die receiving area. A solderpaste mass is interposed between the die receiving area and theelectrode of a die and reflown. That is, the solder paste may bedeposited inside the can and the die placed thereon, or the die may havethe solder paste deposited thereon and placed inside the can. Becausethe frame cannot be wet by the reflown solder (which is in liquidphase), it is contained within the boundary of the frame. As a result,the die is prevented from moving inside the can during the reflowprocess.

In one embodiment of the present invention, the frame is made fromsolder resist material, which is preferably based on a polymer. If so,the frame may be formed through stenciling or drop-on-demand deposition.

In another embodiment of the present invention, the frame may be madefrom a passivation material such as an oxide. If so, in one preferredembodiment, the die receiving area is covered, and the area which is notcovered is rendered passive and unwettable by liquid solder, forexample, by oxidation. The covering is then removed to expose the diereceiving area.

It has been observed that a frame formed according to the presentinvention improves the placement accuracy of the die in a can.Furthermore, it has been observed that the die can be effectivelycentered within the can and aligned with the sidewalls of the can.

In another embodiment, a dielectric material with high thermalabsorption characteristics is deposited on the exterior surface of thecan. The dielectric material may be a polymer which can bedrop-on-demand deposited onto the exterior surface of the can. Toenhance the absorption capability of the dielectric, it may be pigmentedwith dark or black pigments.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of a package according to prior art.

FIG. 2 is another perspective view of the package of FIG. 1.

FIG. 3 is a cross-sectional view of the package of FIG. 1 along line 3-3in FIG. 2.

FIG. 4 shows the package of FIG. 1 as assembled on a circuit board.

FIG. 5 shows a top plan view of the inside of a can.

FIG. 6 shows the can of FIG. 5 after receiving a frame according to thepresent invention.

FIG. 7 shows the can of FIG. 6 after receiving a solder paste mass inthe die receiving area thereof.

FIG. 8 shows can of FIG. 7 after receiving a die.

FIG. 9 shows a perspective view of the can as shown in FIG. 6.

FIG. 10 shows a perspective view of the exterior of a can after beingcoated with a dielectric.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring next to FIGS. 5-8, in a method according to the presentinvention at least interior surface of web portion 13 of can 12 receivesa frame 38 (see FIG. 6). Frame 38 defines a die receiving area 40 insidecan 12 on web portion 13 for receiving a semiconductor die, for example,semiconductor die 14 according to the prior art. Next, according to apreferred embodiment, a solder paste mass 42 is deposited on receivingarea 40 as illustrated by FIG. 7. Thereafter, a semiconductor die isdisposed on solder paste mass 42, and the solder is reflown by heatingthe arrangement to at least the reflow temperature of solder paste mass42. Next, the arrangement is cooled whereby the reflown solder paste ishardened to form a conductive adhesive body 18.

In a process according to an alternative embodiment, solder paste isdeposited on an electrode of a semiconductor die, and the die is placedon receiving area 40 with the solder paste disposed between theelectrode of the die and receiving area 40. Thereafter, the arrangementis subjected to heat in order to reflow the solder paste, followed by acooling step to solidify the reflown solder. Thus, in both embodimentssolder paste is interposed between the electrode of a die and receivingarea 40 prior to the reflow step.

In the preferred embodiment of the present invention, the semiconductordie is a power MOSFET having the same or similar features as that of die14 in a package according to the prior art. Thus, in the preferredembodiment, the drain electrode of a power MOSFET 14 is electrically andmechanically attached to receiving area 40 of web portion 13 of can 12to realize a package similar to the prior art package shown by FIGS.1-4. It should be noted, however, that in a process according to thepresent invention, an IGBT, a diode, or the like semiconductor die canbe used without deviating from the scope and the spirit of the presentinvention.

In one embodiment of the present invention, frame 38 may be printed onweb portion 13 of can 12 through stencil printing. In another embodimentof the present invention, drop-on-demand deposition may be used to printsolder mask frame 38. U.S. patent application Ser. No. 11/367,725,assigned to the assignee of the present invention and incorporatedherein by reference discloses drop-on-demand deposition as applied inthe semiconductor package fabrication arts.

Essentially, frame 38 can be printed by a print head usingdrop-on-demand deposition as disclosed in U.S. patent application Ser.No. 11/367,725.

Drop-on-demand deposition is advantageous in that it can be used toprint images accurately on irregular (i.e. not flat) surfaces.Furthermore, compared to other methods (e.g. stenciling, or blanketdeposition/photoimaging) drop-on-demand deposition is less wasteful, andrequires fewer steps.

A process according to the present invention is particularly useful whenhigh positional accuracies are required. Specifically, when solder pastemass 42 is reflown, the liquid solder reticulates from solder resistframe 38 whereby the liquid solder spread can be contained. That is, dueto the surface tension the solder liquid will only occupy the aperturewithin frame 38. It also draws the solderable surface of the die to itwhereby the die position can be held within the perimeter of frame 38.

A process according to the present invention may also prevent solderspread or shorting from stray solder in some applications, and is,therefore, useful as an aid for manufacturing and customer applications.

In the preferred embodiment, a solder resist material is used to formframe 38. A suitable solder resist may be a polymer based solder resistthat is capable of deposition by drop-on-demand deposition.

According to an alternative embodiment, the polymer-based solder resistframe 38 can be replaced with a passivated surface inside can 12. Forexample, receiving area 40 may be masked with an oxidation retardantsubstance, and the remaining area can be oxidized to act as apassivation and/or solder mask. Thereafter, the oxidation retardant maskcan be removed leaving receiving area 40 surrounded by a passivatedframe which is treated to act as a solder resist or solder stop.

Referring to FIG. 10, in another embodiment of the present invention,the exterior surface of can 12 may be coated with a high thermalabsorption dielectric 44. Preferably, dielectric 44 is a polymer. As afurther enhancement dielectric 44 may include pigments to furtherimprove the thermal performance thereof. Specifically, in a prior artpackage can 12 is coated with a reflective material such as silver,which has low rate of infra-red absorption. Dielectric 44, andspecifically dielectric 44 pigmented with black or dark pigments willincrease infra-red absorption. Increased infra-red absorption can beadvantageous when an older, less efficient reflow oven is used to reflowsolder paste mass 42. Dielectric 44 can be deposited through any knownmethod, including drop-on-demand deposition.

Can 12 in a method according to the present invention may be made fromcopper, or a copper alloy, and is preferably coated with gold or silver.Similar to the prior art, can 12 may include web portion 13, surroundingwall 15, and rails 32. It should be noted that dielectric 44 ispreferably applied before the application of solder mask frame 38 andattachment of the die.

A process according to the present invention is further advantageous inthat it is fully customizable. That is, the process parameters can bechanged to achieve any particular end result without substantiallychanging the fundamental aspects of the process.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. A process for manufacturing a semiconductor package, comprising:forming a frame comprised of solder resist inside a conductive can todefine a receiving area inside said can for receiving a semiconductordevice, said frame being unwettable by solder in liquid phase; providinga semiconductor device having a first power electrode on one surfacethereof; interposing a solder paste mass between said surface area andsaid first power electrode; reflowing said solder mass; and solidifyingsaid solder mass, wherein said conductive can includes a web portion,anda wall surrounding said web portion , and wherein said frame is formedon said web portion between said wall and said receiving area, whereinsaid frame surrounds said receiving area.
 2. The process of claim 1,wherein said semiconductor device is a power semiconductor device. 3.The process of claim 1, wherein said semiconductor device is a powerMOSFET.
 4. The process of claim 1, wherein said semiconductor device isan IGBT.
 5. The process of claim 1, wherein said semiconductor device isa diode.
 6. The process of claim 1, wherein said frame is drop-on-demanddeposited.
 7. The process of claim 1, wherein said frame is stencilprinted.
 8. The process of claim 1, wherein said conductive can iscomprised of copper.
 9. The process of claim 1, wherein said conductivecan is plated with silver or gold.
 10. The process of claim 1, whereinsaid conductive can includes an exterior surface coated with a highthermal absorption dielectric.
 11. The process of claim 10, wherein saiddielectric is a polymer.
 12. The process of claim 10, wherein saiddielectric is a pigmented polymer.
 13. The process of claim 1, whereinsaid frame is comprised of a passivation material.
 14. The process ofclaim 13, wherein said passivation material is an oxide.
 15. The processof claim 1, further comprising drop-on-demand depositing a high thermalabsorption dielectric on the exterior surface of said conductive can.16. The process of claim 15, wherein said thermal absorption dielectricis a high thermal conductivity polymer.